Such devices are used for laboratory filters and technical and industrial filters. They have lengths of a few centimeters up to approximately one meter. The media can be air or liquids. They are used in the production of pharmaceutical products, food products, high-purity water, dielectric oil, chemicals, cosmetics, solvents, electroplating solutions, resins, fats and oils, radioactive waste, paints and varnishes, inks, air and gas, steam, acids, liquified gases, syrup, cellulose solutions, hydraulic oils, polymers, adhesives, resins, lubricants, fuels, lubricating oils, untreated water, condensation returns, light oils and service water.
Depending on the particular use, the filter finenesses can be in the range of 1 to 500 microns.
In approximately 80% of cases, the outer face of the filter cartridge is the inflow surface, and the inner face forms the flow-off surface.
In the remaining cases, the flow passes through the filter cartridge from the inside outwards, for example in the filtering of compressed air. Here, the water then runs off on the outer face of the filter cartridge. If the flow passes through from the inside outwards, the filter cartridge can also have the form of a filter bag which is inserted in a metal cartridge having wall perforations. Filter devices of this type, but usually without the disk-shaped adaptor, are described, for example, in the cataglog G120.5 G entitled "Industrial Filters" of Messrs. AMF Deutschland GmbH, Cuno Filter, 6200 Weisbaden, Konradinstrasse. The disk-shaped adaptor mentioned at the beginning of this specification is used in cases where the filter cartridges are closed at their free end. The adaptor then retains the filter cartridges and has the necessary orifices so that the medium can flow in on one side and flow off on the other side.
The filter device usually has a head which can be detached from the adaptor. However, it is also possible for the adaptor to be in one piece with the head.
Depending on the particular use, the known filter devices can have the following disadvantages:
a. For example in the filtration of domestic water and even more so in sterile filtration, the filter cartridges should not be touched by hand. For example, the pressure on the surface of beer in beer storage vessels is of course generated by pure compressed air. But sterility also plays an important part in the pharmaceutical industry, food production, etc. Although manufacturers enclose plastic gloves with their deliveries so that the filter cartridges supplied sterile remain sterile, nevertheless it is annoying to have to put on plastic gloves. But even if plastic gloves are worn, the filter cartridges still come in contact with the impure ambient air.
b. During filtration, filtrate settles on the inflow surface in the course of time. Consequently, the filter cartidges eventually become worn out.
Now when the filter cartridges are changed, it is never possible to prevent filtrate from falling into the bowl from the filter cartridges. The filter bowl then has to be washed. It is also necessary to wash the filter cartridge if this is to be re-used or if it cannot be discarded along with the filtrate adhering to it.
The adaptor will also have to be washed at the same time. It is almost always necessary to wash the filter head too, because filtrate can also penetrate into this.
This washing process is extremely unpleasant. Solvent is usually needed for it. This means that large volumes of solvent are present and that the persons employed in washing have to wear breathing masks. Waste disposal is very difficult here.
c. The filter bowls not only have to withstand pressure, but must also withstand attack by the media. This means that it is not only necessary to select the materials for the filter bowls according to the pressure anticipated, but that special materials are frequently required for this purpose. For example, seawater-proof special steel is necessary if seawater is to be filtered. The same of course applies to acids, lyes, or other more or less aggressive media.
d. It is sometimes desirable to recover materials such as, for example, metals or precious metals, from the concentrated filtrate. These materials have indeed per se settled on the filter cartridge in a high concentration. However, when the latter is exchanged, it is impossible to prevent at least some of the filtrate from floating, plopping or otherwise passing back into the filter bowl, so that the materials have to be concentrated once more.
e. As regards those materials classed as dangerous, such as, for example, radioactive materials, the filter bowl comes in contact with these and also has to be discarded.
f. Because the exchange lasts a long time, washing takes a long time and a great amount of time is taken up in further work, there is a long outage time. This means that filter systems have to be oversized if they have to operate continuously.